The $87 million that employees initially allotted to the company IRA plan in 2012 swelled to $664 million by the end of 2017, including $574 million in Medallion, the filings show. In turn, the IRA money -- held by about 250 employees -- grew to more than 4 percent of Medallion’s gross assets from about 1 percent five years earlier.

“Roth IRAs are hugely tax advantaged,” said Len Burman, a co-founder of the Tax Policy Center, a joint project of the Urban Institute and the Brookings Institution. “If you are expecting to earn a rate of return of 20 percent or higher, it turns out to be a really good tax shelter.”

This is still restricted to employees of the fund, but this is so good that if it’s made available outside, it might become one of the primary vehicles for the average investor.